This invention concerns an aircraft take-off performance indicator system.
In most countries regulations stipulate minimum accelerations during the take-off run of specified transport aircraft. In addition, for every take-off a decision point is established in terms of air speed, known as the V.sub.1 speed. Assuming that the aircraft adopts its minimum legal acceleration characteristic for any given take-off, this V.sub.1 speed corresponds to a particular point along the runway, known as the V.sub.1 distance. If a power unit fails before this decision point is reached then, by regulation, the take-off must be aborted. If a power unit fails at the decision point the pilot has discretion and may either continue or abort the take-off depending upon his own judgment of the situation. If a power unit fails after this point the take-off shall be capable of, and by regulation must be, continued.
Conventionally, therefore, the pilot at the start of his take-off run notes down the V.sub.1 speed for that take-off and in the event of engine failure compares his actual speed with the V.sub.1 speed to decide whether to continue with the take-off or abort it. There is however a serious danger in this. If for any reason the aircraft does not achieve the minimum acceleration characteristic the V.sub.1 speed will not be reached until after the V.sub.1 distance has been passed. This means that the pilot's decision to abort the take-off will be made at a point further along the runway than has been assumed in the calculations, with possibly dangerous consequences. It will be appreciated that it is quite likely that a power unit which is on the point of failure will give less than its anticipated thrust, so that the danger of such occurrence could be substantial.
Proposals have been made to provide a distance indicator giving the pilot of an aircraft an indication of distance run (or of distance to the end of the runway) from which the pilot can make a decision to abort or continue with take-off, at or at a point near, the V.sub.1 distance. Such arrangements are described in British Pat. No. 1,082,341 and a development thereof in British Pat. No. 1,327,771. In these patents, the V.sub.1 speed and V.sub.1 distance are pre-set by the pilot and an indication is given as to whether the V.sub.1 distance is reached before the V.sub.1 speed is attained, to enable him safely to abort the take-off run.
U.S. Pat. No. 2,922,982 describes a proposal for a continuous take-off performance indicator. In this proposal a sensing finger follows a groove in a plate which is moved during take-off by a drive from the aircraft wheels. The groove represents a specified speed versus distance characteristic. The finger is coupled to an extra pointer on the aircraft's existing air speed indicator, and thus the position of this extra pointer is dependent on the shape of the groove and hence the specified speed/distance characteristic. The pilot can thus compare his actual air speed with the speed determined by the shape of the groove.
The groove in the plate defines a curve which is arbitrary and fixed. Some adjustment is possible by rotating the plate about the origin of the curve so that at a pre-set V.sub.1 distance it gives the required V.sub.1 speed. In addition in the prior U.S. Patent a separate Go/No-Go indication is given at the pre-set V.sub.1 distance as to whether or not the V.sub.1 speed has actually been attained.
In practice the shape of the speed versus distance curve for any given take-off is dependent upon a large number of parameters. These may be divided broadly into the following categories:
(i) fixed characteristics of the aircraft; PA1 (ii) fixed characteristics of the runway, e.g. its length and slope, and height above sea level; PA1 (iii) aircraft and runway parameters individual to any particular take-off, e.g. all-up weight, and weather conditions such as temperature, wind and runway surface conditions; and PA1 (iv) variable aircraft parameters such as flap condition and (possibly) the percentage of maximum attainable thrust which is to be used.
The system of U.S. Pat. No. 2,922,982 takes account only of the fixed aircraft characteristics. It cannot therefore give the pilot an accurate indication for any take-off of how the aircraft's actual performance compares with what he should expect. Furthermore the curve is embodied in mechanical form and the mechanical sensing of this curve is unsatisfactory and likely to lead to errors.
This invention has the object of providing a continuous and accurate indication of take-off performance in which the likelihood of error is reduced.